1. Field of the Invention
Embodiments of the invention relate to doped carbon-sulfur species nanocomposites, as well as methods of making them. Further embodiments relate to objects that may be made from these composites, including lithium-sulfur batteries and battery components.
2. Background of the Related Art
The lithium-sulfur (Li—S) battery has attracted increasing attention as a next-generation energy storage device for plug-in hybrid electric-vehicles and electric vehicles. This is largely due to its extremely high theoretical specific capacity (1672 mA h g−1) and energy density (2600 Wh Kg−1). In addition, sulfur is low-cost, abundantly available, and eco-friendly.
Li—S batteries operate by reaction of sulfur with lithium to form lithium polysulfides (i.e., Li2Sx, 8≦x≦3), lithium disulfide (Li2S2), and finally lithium sulfide (Li2S) during the discharge process, with the reverse occurring during the charge process.
Despite these considerable advantages, practical realization of a Li—S battery is hindered by several issues. First, the low electrical conductivity of sulfur (5×10−30 S cm−1) limits utilization of active materials and leads to a poor capacity.
Second, a significant impediment to widespread adoption of lithium-sulfur batteries is the diffusion of polysulfide, called the “polysulfide shuttle effect.” The shuttle effect results in fast capacity fading and low coulombic efficiency. This occurs when intermediate lithiation compounds, lithium polysulfides, dissolve in organic electrolyte and are deposited on the anode surface. This causes a progressive decrease in coulombic efficiency, loss of active materials, and capacity fading with cycling. Third, electrolyte consumption upon reaction with the Li anode results in capacity fading during the charging/discharging process.
To address these challenges, various strategies have been explored. These include design of new electrolytes, inclusion of protecting layers, and novel sulfur-carbon and conducting polymer cathodes.
Still, the development of high-performance cathodes remains a significant challenge. Many initiatives have synthesized sulfur-carbon nanocomposite cathodes to improve contact between sulfur and carbon. This may result in increased conductivity of the electrodes, leading to enhanced utilization of the active sulfur.
Two main approaches to limit capacity loss upon cycling sulfur-based cathodes have been developed. One approach to circumvent capacity fade has been to tether sulfur within a cathode material with an organic molecular chain. This approach attempts to prevent the sulfur from migrating out of the cathode material and becoming electrochemically useless by using the organic molecular chain to attach to the sulfur and/or sulfur-containing species. Such an approach has been investigated and reported in U.S. Pat. Nos. 4,833,048; 5,162,175; 5,516,598; 5,529,860; 5,601,947; 6,117,590; and 6,309,778. Those patents are incorporated by reference herein.
A second approach to limit the capacity fade of a lithium battery due to sulfur migration from the cathode has been to create confinement and physical sorption between an additive and polysulfides created within the battery system. This approach has been disclosed in U.S. Pat. Nos. 5,532,077; 6,210,831; 6,406,814; and 8,173,302. Those patents are incorporated by reference herein.
Unfortunately, the first approach is limited by the non-conductive nature of the organic molecule chain. The second approach is limited by the weak anchoring effect on polysulfides by physical sorption in conductive matrix.